Cyclobutadiene complexes halogenation

Tetraphenylcyclobutadienepalladium and -nickel complexes and tetra-methylcyclobutadienenickel chloride react readily with nucleophilic reagents to give 7r-cyclobutenyl complexes 12, 30, 31, 65, 91), a reaction reminiscent of those described by Chatt et al. for diene-palladium and diene-platinum halide complexes (Section VI, F). Non-halogen-containing cyclobutadiene complexes, however, appear inert under similar conditions so that this reaction is very dependent on the other ligands present. Some similarity between cyclobutadiene-metal and diene-metal complexes appears to exist but how far the parallel can be drawn remains to be seen. The reactions are fully discussed in Section VI. 

Addition of halogens to planar d complexes is straightforward and does not deserve further comment. However, it should be noted that halogenation can also be achieved by treatment with the carbon tetrahalides (26, 41, 67). The mechanism of this reaction is obscure but a radical path may be involved (104). The fate of the carbon fragment has not been determined but it may be anticipated that carbene like intermediates could be formed in this way. A related process is the halogenation of reactive four-coordinate complexes by vicinal dihalides which are converted into olefins. Only two examples have been described (28,56a). The more interesting involves the formation of the cyclobutadiene complex (XLVI) (56a). 

Cyclobutadiene transition metal complexes were actually synthesized long before cyclobutadiene itself.14 Interestingly, one of these complexes was also a halogen-bridged dimeric nickel complex, as shown on the next page. 

Certain metal carbonyls are very useful for the removal of halogens from polyhalogenated compounds. In these reactions, again, an oxidative addition step should be important. As shown below, carbenes 33>, cyclobutadiene 34>, and benzyne complexes 35> are formed by the dehalogenation reaction with metal carbonyls. 

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